Hi.I remember once asking an American about his past position in a university in a PM, and he got angry with me. Days afterwards, I learned at one of my university classes from a professor that some questions are forbidden in, e.g, Australia. How to learn about theses codes of conduct?

Thanks in advance
I wouldn't call such a question 'forbidden. Much depends on how well you know the person,

You really need to live in a culture to grasp ts taboos in detail. Where I live, broadly speaking, topics to be careful with include sex, religion, money, politics.

How about where you live?

Well, as far as it doesn't get very personal, I think anything is OK. But I don't think anyone will get that angry . Now what should I do now that this inconsistency led to him being irritated?
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Just forget it. I'm sure he has.
He hasn't. He has not answered me since then.
Sorry, I have no idea why asking someone about his past position in a university would be considered offensive.

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Nor have I, but there is an ancient academic joke that goes:

"Why are academic disputes so bitter?"

"Because the stakes are so low."